1. Field of the Invention
The present invention relates to a light quantity adjusting apparatus, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
A stepper (i.e., a one-shot-type projection exposure apparatus) and a scanner (i.e., a scanning-type projection exposure apparatus) can be used to manufacture a semiconductor device. The scanner is operable according to a step-and-scan method that includes moving a reticle and a wafer synchronously relative to a projection optical system and exposing the reticle to light so that a pattern image of the reticle can be transferred onto each shot region of a wafer.
Similar to the one-shot-type projection exposure apparatus, the scanning-type projection exposure apparatus requires an exposure amount control mechanism that can minimize unevenness of illuminance so that an exposure amount (integrated exposure amount) at each point of each shot region on the wafer can be equalized to a target exposure amount.
As discussed in Japanese Patent Application Laid-Open No. 8-236439, a conventional system can adjust an exposure amount by controlling at least one of a light quantity adjustment unit (e.g., a neutral density filter) or the pulse laser oscillation frequency. FIG. 6 illustrates a conventional light quantity adjustment unit that includes a plurality of neutral density (ND) filters 2 which are mutually different in transmissivity and disposed on a turret 1. The light quantity adjustment unit can switch the ND filter 2 positioned across the optical axis to select a desirable transmissivity (i.e., adjust the quantity of light).
An exposure control attaining a required accuracy can be realized by effectively using the above-described adjustment methods. If the above-described adjustment methods are both used, a pulse control according to the beam attenuation effect can be realized. An accurate exposure amount control can be realized based on a pulse width modulation.
Furthermore, as illustrated in FIG. 7, a conventional system discussed in Japanese Patent Application Laid-Open No. 5-251310 includes a total of five (i.e., a plurality of) ND filters 4 disposed on each of two turrets 5 which are serially disposed on an optical axis 3 of illumination light. This conventional system can realize a speedy light quantity adjustment by changing a combination of the ND filters 4 of two turrets 5 to attenuate the quantity of light, compared to a system using a single turret including a plurality of ND filters.
More specifically, the conventional system of FIG. 7 can realize a total of 25 (=5×5) beam attenuation levels by changing a combination of the ND filters 4 of the turrets 5. It is needless to say that a large-scale turret is required if twenty-five ND filters are disposed on the same turret. A complicated hardware arrangement, including a mechanism for speedily switching numerous ND filters, is required.
FIG. 8 illustrates a conventional light quantity adjustment (discrete light quality levels) realized by switching ND filters. In FIG. 8, the abscissa axis represents an ND filter number, and the longitudinal axis represents the laser beam transmissivity. To simplify the description, it is now presumed that ND filters are serially disposed to constitute a turret of single stage. The transmissivity decreases when the ND filter number increases.
As apparent from FIG. 8, the change of the laser beam transmissivity is stepwise. In other words, this conventional system cannot realize any intermediate transmissivity between fixed transmissivities unique to respective ND filters. For example, the system cannot realize an intermediate transmissivity “a” [%] between two transmissivities of the second and third filters. In this case, the system selects the second filter because the transmissivity of the second filter is closest to the intermediate transmissivity “a.”
On the other hand, a conventional system can control an exposure scanning speed or the oscillation frequency of a light source laser to accurately control an exposure amount. However, if the scanning speed is high, the apparatus requires a high-performance driving mechanism and therefore the apparatus cost increases. If the scanning speed is low, the throughput may be insufficient.
Furthermore, if the oscillation frequency of a light source laser is high, the life of the laser may decrease. On the other hand, if the oscillation frequency of the light source laser is low, unevenness of illuminance may occur.
To solve the above-described problems, the following method may be employed to decrease a difference between a desired transmissivity and a realizable transmissivity. The method uses an increased number of stages for a plurality of ND filters to decrease the gap between unique transmissivities (i.e., stepwise or discrete transmissivities in the longitudinal axis).
However, this method requires a great number of ND filters and a complicated hardware arrangement. The cost increases correspondingly. Therefore, an appropriate optimization is required not only to decrease the cost and the apparatus scale but also to increase the accuracy in the light quantity adjustment. As a result, the number of stages is limited. The gap between the stepwise or discrete transmissivities in the longitudinal axis cannot be decreased to an intended level.